Fixes hyperhive#3014.
docs/README.md is a genuine hand-written index for the docs/ tree -
task-oriented reading paths grouped by topic, covering every top-level
doc plus the three subdirectories that already have their own landing
page (web-ui/, tools/, turn-loop/). Adapted from CLAUDE.md's existing
"Reading paths" section (already curated and kept current) rather than
written from scratch, reorganized into headed groups since this is a
landing page, not a flat reference list. Explicitly covers
docs/tools/matrix.md and docs/github.md, per the note on hyperhive#3014
about the content that moved out of README.md's deleted section on
PR #3006 not going undiscoverable.
Top-level README.md's reading-path table (enumerating every doc)
replaced with a single prominent docs-site link, per mara's "replace
docs links table in readme md with a prominent link to the docs
(public host and git relative path)" - both forms present (rendered
site URL, git-relative docs/ path).
Verified every link in docs/README.md resolves to a real file (33
files checked). nix fmt clean, tracker-tag grep clean.
Companion to hyperhive/website#47/#48 - this is what that PRs "index.html
half" needs to exist before it can render, per atlas's note on #48.
